Optical measuring instruments are typically utilized in the microelectronic industry for non-contact, non destructive measurement of the thickness of thin films. Two main systems are utilized, spectrophotometers (or reflectometers) and ellipsometers. The following U.S. patents represent the prior art:                For ellipsometers: U.S. Pat. Nos. 5,166,752, 5,061,072, 5,042,951, 4,957,368, 4,681,450, 4,653,924, 4,647,207 and 4,416,855.        For spectrophotometers: U.S. Pat. Nos. 5,181,080, 5,159,412, 5,120,966, 4,999,014 and 4,585,348.        
The two prior art systems are illustrated in FIGS. 1A and 1B, respectively, to which reference is now made. The spectrophotometer utilizes the fact that light beams reflected off thin film boundaries, will interfere one with another. Specifically, the spectrophotometer of FIG. 1A measures the reflectance of selected points of a sample 10 as a function of the light wavelength, usually in the visible or near UV spectral ranges. Computer analysis of the detected spectral reflection function, especially its minima and maxima, provides the thickness, and in some cases, also the index of refraction of the measured film.
The spectrophotometer typically includes a transmitter 12 with a light source and appropriate optics, a beam splitter 14, an objective lens 16, a tube lens 18 and a receiver 20 which includes optical and electronic means for measurement of light intensity as a function of the input light wavelength. The transmitter 12 produces a collimated light beam 22 which is deflected by the beam splitter 14 and focused on the sample 10 by the objective lens 16. The reflected beam, labeled 24, is collected by the microscope imaging optics (lenses 16 and 18) on to a spectroscopic measurement unit within the receiver 20.
In order to measure a multiplicity of points on the sample 10, sample 10 is placed on an x-y stage 26. X-Y stage 26 is typically very precise and heavy and, as a result, moves very slowly.
The spectrophotometers have difficulty measuring structures with very small reflectance, such as thin films on glass substrates, because the relatively low brightness of traditional white light sources does not provide a sufficient signal-to-noise ration (SNR). Spectrophotometers also have difficulty measuring films with unknown or unrepeatable dispersions of optical constants, such as amorphous silicon.
Despite these limitations, the spectral photometry method is at present widely used in industry because the instrumentation for this method is easily combined with optical microscopes and can utilize conventional microscope optics.
Ellipsometers measure changes in the polarization of light caused by reflectance from the test surface. These changes, characterized as amplitude and phase changes, are very sensitive to the thickness and optical properties of thin films.
A prior art ellipsometer is illustrated in FIG. 1B. It includes a transmitter 30 which includes a light source and appropriate optics, a polarizer 32, an optional compensator (phase retarder) 34, an analyzer 36 and a receiver 38 with a photo-detector and appropriate electronics. The polarizer 32 polarizes the light beam 40 produced by light source 30. The reflected light beam, labeled 42, passes through the analyzer 36 before reaching the receiver 38. If the compensator 34 is used, it may be located either between the polarizer 32 and the test sample 10 or between the sample 10 and the analyzer 36.
The ellipsometric method requires oblique illumination, i.e. an angle of incidence Θ between an incident light beam 40 and a normal 44 to the sample 10 must be greater than zero. The angle between a reflected light beam 42 and the normal 44 is equal to the angle of incidence Θ. The angle of incidence Θ should be close to the Brewster angle ΘB of the substrate. In practice, the angle of incidence Θ ranges from 45° to 70°.
Because ellipsometers measure two polarization parameters (amplitude and phase), both of which are independent of the light intensity, they are quite accurate and can also measure ultra thin films of the size of 0-100Å. However, since ellipsometers require oblique illumination as well as a highly collimated light beam, their use for high spatial resolution measurements in dense patterned structures is rather difficult
There are two basic types of fully automated ellipsometers. Null-ellipsometers (NE) provide the most accurate thickness measurements but they require at least several seconds per measuring point. Rotating-analyzer ellipsometers (RAE) provide very high speed measurements (portions of a second per measuring point), but their sensitivity and accuracy are usually less than those of null ellipsometers.
For all of the prior art instruments, the opto-mechanical apparatus is complicated, large and heavy, and thus, the x-y stage 26 is translated between measurement points, coming to a complete stop before measurement begins. The time between measurements depends on the mass of the x-y stage 26 and on the positioning accuracy requirements and may take at least several seconds (sometimes up to several tens of seconds). This limits the speed with which a thickness mapping can occur, especially during inspection of large size substrates such as 8″ VLSI silicon wafers, 18″×18″ LCD glass panels, etc.
The footprint, or space on the floor which each machine utilizes, is typically at least twice the size of the x-y stage 26 due to its translation.
Furthermore, the prior art measuring devices are utilized for measuring once a deposition process has been completed. They cannot be utilized for in-process control, since wafer handling and other mechanical movements are not allowed within a vacuum chamber.
Other measuring instruments are also known, one of which is described in U.S. Pat No. 4,826,321. The '321 patent presents a system similar to an ellipsometer. However, in this system, a mirror is utilized to direct a plane polarized laser beam to the thin film surface at the exact Brewster angle of the substrate on which the thin film lies.